The invention concerns an optically variable element, in particular an optically variable safeguard element for safeguarding banknotes, credit cards and the like, wherein the optically variable element has a thin film layer succession with at least one spacer layer for producing color shifts by means of interference. The invention further concerns a security product having such an optically variable element, a foil, in particular an embossing foil or a laminating foil with such an optically variable element and a process for producing such an optically variable element.
Optically variable elements are frequently used to make it difficult to misuse documents or copy documents and if possible to prevent that from happening. Thus optically variable elements are frequently used for safeguarding banknotes, identity papers, credit cards, cash cards and the like. It is further possible for them to be applied to articles and to be used for detecting authenticity of goods.
In order to make it difficult to copy optically variable elements, it is known for an optically variable element to be provided with a thin film layer succession which produces shifts by means of interference, in dependence on the viewing angle.
Thus, WO 01/03945 A1 describes a security product having a transparent substrate, to one side of which is applied a thin film which produces a perceptible color shift in dependence on the change in the viewing angle. The thin film comprises an absorption layer which is applied to the transparent substrate. The thin film further comprises a dielectric layer which is applied to the absorption layer. The absorption layer includes a material which consists of one of the following materials or an alloy of the following materials: chromium, nickel, palladium, titanium, cobalt, iron, tungsten, molybdenum, iron oxide or carbon. The dielectric layer comprises one of the following materials or a combination of the following materials: silicon oxide, aluminum oxide, magnesium fluoride, aluminum fluoride, barium fluoride, calcium fluoride or lithium fluoride. In order further to increase the level of safeguard against copying, WO 01/03945 A1 further proposes embossing a diffraction pattern on the side of the transparent substrate, which is in opposite relationship to the thin film layer.
EP 0 660 262 B1 proposes building up a thin film layer succession comprising a plurality of successive layers with differing refractive indices. Those layers involve layers of ceramic material which are successively applied to a carrier substrate by vapor deposition. Thus firstly a release layer comprising thermoplastic acrylic resin to which silicone oil is added is applied to a polyester film which is 12 μm in thickness. Then a magnesium fluoride layer which has a low refractive index and a zinc sulfide layer which has a high refractive index are alternately applied by vapor deposition. In total in that case five such layers are successively applied by vapor deposition. The total layer thickness of the thin film layer succession formed in that way is 1 μm in that case. An adhesive layer is then applied to the thin film layer succession.
EP 0 660 262 B1 further proposes engraving a pattern into the release layer. That provides that, upon transfer of the thin film layer succession on to an article or a document, the thin film layer succession is released from the carrier material only in the region in which the release layer is present. The thin film layer remains adhering to the carrier material in the other regions, and thus tears away. That provides that only parts of the thin film layer are stuck on the article or document. The same effect can also be achieved if the adhesive layer is applied in a pattern configuration.
That provides that, in a first partial region, there is a different number of thin film layers than in a second region, and thus the total layer thickness of the thin film layer succession varies between 1 μm (five thin film layers) and 0 (no thin film layers). That ‘region-wise application’ of thin film layers to an article can produce on the article a pattern which can be electronically detected and evaluated by means of an evaluation device.
WO 02/00445A1 describes an optically variable element comprising a plurality of layers which are arranged throughout in mutually superposed relationship. The optically variable element has on the one hand a thin film layer succession. That thin film layer succession produces the effect of a color change which is dependent on the viewing angle, by means of interference. In addition the optically variable element has a replication layer into which diffractive relief structures are embossed. Those diffractive structures make it possible to achieve diffraction effects by which holograms and the like can be represented.
In that respect, in regard to production procedure, firstly the thin film layer succession is applied to the replication layer and then the relief structure is embossed thereon.
WO 02/00445 A1 proposes that the thin film layer succession is built up from an absorption layer and a spacer layer. It is further proposed that a prefabricated thin film layer succession is provided with an embossable lacquer and then relief structures are embossed into that lacquer. As an alternative thereto it is stated that the prefabricated thin film layer succession can be glued to prefabricated microstructures.